Pointers and strings, failing on EOF - c

Question

Code rewritten to be more clear
void indexe(char * line, unsigned ref) {
unsigned i = 0;
char word[128]; //(1)
//char * word; //(2)
while(*line) {
if(isalpha(*line))
word[i++] = *line; //(1)
//*word++ = *line; //(2)
*line++;
}
}
int main(int argc, const char * argv[]) {
char line[128];
FILE * f = fopen(argv[1], "r");
unsigned x = 0;
while (fgets(line, 128, f)){
indexe(line, ++x);
}
fclose(f);
return 0;
}
Hello, I have tried the two above combinations:
word[] -> word[i++]
*word -> *word++
The whole thing works flawlessly, except when reaching EOF, in which case the pointers syntax fails with a segmentation fault, but not the array syntax.
I am a C total beginner, could someone explain in beginner terms what happens here, and maybe suggest a solution to fix the pointer syntax? (But mostly explain, please)

Answer 1

This version, as posted, is fine:
void indexe(char * line, unsigned ref) {
unsigned i = 0;
char word[128]; //(1)
//char * word; //(2)
while(*line) {
if(isalpha(*line))
word[i++] = *line; //(1)
//*word++ = *line; //(2)
*line++;
}
}
However, if you recomment the code to use the lines marked //(2) instead, you have this:
char * word; //(2
*word++ = *line; //(2)
Which is simply a case of writing to a pointer you haven't initialised with allocated memory. This isn't allowed. You need to keep it as an array, or use something like malloc to reserve storage. If you want to write the function without using an array at all, the code would be:
char *word = malloc(128); // reserve 128 bytes
if (word == NULL) { // these checks are important
fprintf(stderr, "Cannot allocate memory!");
exit(EXIT_FAILURE);
}
...other stuff...
free(word);
Note also that:
*line++;
increments line, but dereferences it (before it's incremented) for no reason.

Answer 2

The pointer syntax fails because you have defined a pointer char * word; but you have not set it to point to any data - the memory you are pointing to can be anywhere. So, when you execute the following statement:
*word++ = *line;
You are storing the value pointed to by line in the value pointed to by word. Unfortunately, you do not know where word is pointing. As #teppic pointed out, you're writing to a pointer that has not been initialized to allocated memory.
You could malloc that memory as #teppic previously pointed out. You could also do the following:
char reserve[128];
char * word = reserve; // could also have used &reserve[0]
Hope that helps!